'Deep Impact' Impact

CAPE CANAVERAL, Fla. -- A NASA spacecraft with a Hollywood name -- Deep Impact -- blasted off Wednesday on a mission to smash a hole in a comet and give scientists a glimpse of the frozen primordial ingredients of the solar system.

With a launch window only one second long, Deep Impact rocketed away at the designated moment on a six-month, 268-million-mile journey to Comet Tempel 1. It will be a one-way trip that NASA hopes will reach a cataclysmic end on the Fourth of July.

Scientists are counting on Deep Impact to carve out a crater in Comet Tempel 1 that could almost swallow the Roman Coliseum. It will be humans' first look into the heart of a comet, a celestial snowball still containing the original building blocks of the sun and the planets.

Because of the relative speed of the two objects at the moment of impact -- 23,000 mph -- no explosives are needed for the job. The force of the smashup will be equivalent to 4 1/2 tons of TNT, creating a flash that just might be visible in the dark sky by the naked eye in one spectacular Fourth of July fireworks display.

Nothing like this has ever been attempted before.

Little is known about Comet Tempel 1, other than that it is an icy, rocky body about nine miles long and three miles wide.

The comet will be more than 80 million miles from Earth when the collision takes place -- on the sunlit side of the comet, NASA hopes, in order to ensure good viewing by spacecraft cameras and observatories. The resulting crater is expected to be two to 14 stories deep, and perhaps 300 feet in diameter.

Deep Impact is carrying the most powerful telescope ever sent into deep space. It will remain with the mother ship when the copper-fortified impactor springs free the day before the comet strike, and will observe the event from a safe 300 miles away.

NASA space telescopes like the Hubble will also watch the collision, along with ground observatories and amateur astronomers. The impactor will have a camera, too, which will snap pictures virtually all the way in.

The entire mission costs $330 million, all the way through the grand finale.